This invention relates to the control of recovery furnace of the Kraft recovery process.
In the production of paper pulp, cellulosic fibrous material, generally wood chips, is digested in an NaOH-containing solution (white liquor). This digestion yields pulp and, as a by-product, black liquor. The Kraft recovery process is widely used to recover, from the black liquor, chemicals used in the production of paper pulp and recycle these chemicals to the pulping process. Briefly, black liquor is concentrated and then combusted in the recovery furnace to produce smelt. In a dissolving tank, the smelt is dissolved in weak wash solution to form green liquor, the primary component of which is Na.sub.2 CO.sub.3. The green liquor is then reacted with lime (CaO) in a causticizing reaction to convert Na.sub.2 CO.sub.3 to NaOH. The reacted mixture then passes to a clarifier which separates the liquid phase, which is essentially the white liquor, from the solid phase which is primarily CaCO.sub.3 and is referred to as lime mud. The white liquor is recycled and used for digestion of wood chips at the beginning of the cycle. The lime mud is washed to remove traces of white liquor prior to being passed to a lime kiln where CaO is regenerated. The overflow water from the washing of the lime mud is the weak wash used to dissolve the smelt in the dissolving tank.
The recovery furnace is an important part of the Kraft process, since the smelt produced in the furnace is the source of all recycled compounds. Efficient recovery furnace operation results in both reduced emissions and increased recovery of inorganic solids.